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Magnetic resonance imaging (MRI) is a powerful technique due, in part, to
its ability to "see" below surfaces with true three-dimensional resolution.
The main disadvantage of MRI is its insensitivity. Even the best
inductively detected MRI microscopy requires a net nuclear polarization on
the order of 100 million spins per voxel, which limits its spatial
resolution to about 3 micrometers. Using ultrasensitive force detection,
magnetic resonance force microscopy (MRFM) can greatly improve the
sensitivity of nuclear spin detection. We describe recent work
demonstrating two dimensional MRFM imaging of fluorine-19 nuclei in a
calcium fluoride sample with 90 nm resolution. The baseline sensitivity
corresponds to a net polarization of about 200 nuclear spins. As part of
this work, we have developed magnetic tips that produce field gradients in
excess of 1.4 million tesla per meter (14 gauss per nanometer), developed
improved methods for manipulating nuclear spins and implemented a method of
mitigating spin noise in statistically polarized spin ensembles. Prospects
and challenges of extending MRFM to single nuclear spins will also be
discussed.
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